1. Field of the Invention
The present disclosure generally relates to the production and/or therapeutic use of recombinant type 1 ribosome inhibiting proteins for the treatment of cancer and infectious diseases.
2. Description of the Relevant Art
Ribosome Inactivating Proteins (RIPs), found widely among plants, have been used for their medicinal properties since ancient times. Cucurmosin is an example of a Type 1 RIP. Type 1 RIPs are monomers of about 30 kilodalton (kDa) whereas Type 2 RIPs are heterodimers. Type 1 RIPs tend to have less toxicity than the Type 2 RIPs for normal cells. Cucurmosin is isolated from Cucurbita moschata (pumpkin) seeds—something that is naturally consumed by humans, suggesting a low innate toxicity. Cucurmosin has selective toxicity for tumor cells compared to non-cancer cells in culture via the induction of apoptosis. RIPs such as cucurmosin have therapeutic potential for diseases in addition to cancers including use as an antiviral agent and an antifungal agent.
RIPs are a large family of proteins, generally present in one or more tissues of individual plants. They can be divided into three families based on the number and organization of protein subunits. Type I RIPs (>60 members) contain a single chain of 26-31 kDa. Type II RIPs (˜15 cloned members), which include the relatively well-known ricin compound, possess two disulfide-linked chains—an enzymatic A chain and a lectinic B chain enabling the protein to effectively interact with a broad range of cell types. Type III RIPs are an alternative, non-classical group that are much less characterized than type I or II versions and contain either internal repressive peptides or additional protein chains of unknown function. RIP expression in its natural host is generally stimulated by a variety of environmental stresses upon the plant, including viral infection. Expression is often observed during cell senescence. These observations suggest that RIPs are part of an innate defense mechanism of plants to ensure appropriate cell growth, a hypothesis that emphasizes their potential as possible anti-tumor and anti-infectious disease drugs in humans.
RIPs inhibit protein synthesis. Through interactions with a variety of ribosomal proteins, RIPs target the ribosome where they specifically act as an N-glycosidase and remove adenosine 4324 from the conserved ‘sarcin/ricin’ loop in 28S ribosomal RNA (rRNA). In addition to this rRNA cleavage activity, several other enzymatic activities have been associated with RIPs. RIPs can specifically remove adenine from the ADP-ribose chain of the Poly (ADP-ribose) Polymerase (PARP) protein, a factor involved in DNA repair. Two RIPs have also been implicated in DNA damage, suggesting caspase activation leading to apoptosis. Some RIPs have been associated with glycosidase-independent nuclease activities as well as superoxide dismutase/antioxidant activity. Interaction with the 5′ cap of mRNAs has been reported which is of importance since the eukaryotic cap binding protein eIF4E may transform cells, and translational defects associated with such transformations are often associated with certain cancers (e.g., breast cancers). In summary, although their rRNA modification activity is best characterized, RIPs appear to have additional activities in cells contributing to their biological effects.
RIPs may protect plants from a variety of infectious diseases (e.g., viruses). Introducing the RIP genes into commercially relevant crops may provide protection against some of these diseases.